1. Field of the Invention
The present invention relates to transmission systems and a transmission apparatus, and in particular, to a transmission system and a transmission apparatus for transmitting a signal on an optical access network.
2. Description of the Related Art
In recent years, with widespread use of the Internet, information communication networks have become widely used in homes and companies. Regarding optical access networks, a PON (passive optical network) has come to be widely employed as an optical access system in order to provide a higher-speed, large-capacity service.
FIG. 6 is a block diagram showing the configuration of a PON system 5. The PON system 5 includes an OLT (optical line terminal) 51 provided in a station, a star coupler 52 for performing optical demultiplication and multiplication, and ONUs (optical network units) 53-1 to 53-n provided for subscribers.
The OLT 51 is connected to the ONUs 53-1 to 53-n via the star coupler 52 with optical fibers F in a one-to-n relationship, and optical packet communication is performed between the OLT 51 and the ONUs 53-1 to 53-n. For example, when the OLT 51 transmits 2.5-GHz optical packets as downstream signals, the optical packets are distributed by the star coupler 52 to the ONUs 53-1 to 53-n, and are received by the ONUs 53-1 to 53-n. 
Japanese Unexamined Patent Application Publication No. 2005-33537 (paragraph numbers [0017] to [0026], FIG. 1) proposes, as a PON system technology of the related art, a technology in which various terminal apparatuses having different speed series can be allocated in the same transmission interval of the PON.
In the PON system 5 shown in FIG. 6, when the OLT 51 sends signals having both a low bit rate and a high bit rate, one ONU, which only responds to a low bit rate, cannot perform normal reception processing. Accordingly, in the PON system of the related art, it is difficult to perform communication by using differing transmission bit rates.
FIG. 7 is an illustration of signal reception processing of an ONU. When an ONU receives an optical packet, the ONU uses CDR (clock data recovery) to perform clock extraction, and reproduces transmitted data.
In addition, a frame transmitted from an OLT includes frame-synchronization managing information and data. In FIG. 7, for example, frame-synchronization managing information 6-1, and data 6-2, 6-3, and 6-5 each have a low bit rate, and data 6-4 has a high bit rate.
When an ONU having a CDR portion for a low bit rate receives the signals shown in FIG. 7, the ONU performs data reception processing while extracting clocks from the frame-synchronization managing information 6-1, and the data 6-2 and 6-3 in accordance with the low bit rate. However, when the ONU receives the data 6-4 having a high speed, the CDR portion enters a free running state. Accordingly, if, after that, the ONU receives low-speed data, clock synchronization becomes unstable. Since, in reception processing performed on the data 6-5, data reproduction is performed at unstable clocks, communication quality deteriorates.
In other words, in the PON system of the related art, when communication is performed by using differing bit rates, deterioration occurs in communication quality. Therefore, when a subscriber requests to upgrade the communication speed, ONUs of all subscribers need to be modified.
By way of example, in a case in which, in FIG. 6, the ONUs 53-1 to 53-n initially comply with 2.5-GHz communication, when it is requested that only the ONU 53-1 be able to respond to 10-GHz high-speed signals, 10-GHz high-speed signals transmitted from the OLT 51 are equally distributed to each ONU by the star coupler 52. Thus, in fact, the other ONUs 53-2 to 53-n also need to be modified to be capable of responding to 10-GHz high-speed signals. However, a necessary communication speed differs for each subscriber. For a subscriber who does not wish for upgrade, upgrade results in an increase in cost.